The discovery this summer of dishwasher-inhabiting microbes resistant to vast ranges of temperature and acidity got us wondering: What other places in your household are breeding grounds for bugs?

Dishwasher

Dishwasher

You may think you live alone or with roommates, family or friends. But University of Colorado microbiologist Norman Pace reminds us that we also share our homes with billions of microorganisms (that are mostly benign). However, improved sampling techniques are allowing researchers to better isolate and identify microorganisms like fungi and bacteria in our houses—and they're finding potentially dangerous ones in new and unexpected places.

In June, for example, researchers in Slovenia reported that they'd found several rare species of fungi, including some that are known to infect humans, living in the rubber door seals of household dishwashers. The team swabbed 189 dishwashers on six continents and found that 62 percent of them harbored fungi. Of those, just more than half were growing Exophiala dermatitidis and Exophiala phaeomuriformis, two species of black yeast that can cause systemic infections and colonize the lungs of people with conditions like cystic fibrosis.

Nina Gunde-Cimerman, who led the study, had thought the conditions in dishwashers would be too harsh for microorganisms—they'd have to put up with abrasive soaps, bursts of water at temperatures up to 175 degrees Fahrenheit and dry spells in between dishwasher runs. But it turns out that yeasts, which are very rare in nature, love the extreme conditions like those in the dishwasher. When the researchers tested the fungi in the lab, the fungi managed to survive a wide range of temperatures, high salinity and very high pH, a condition that detergent creates. And because dishwashers are home to a steady supply of nutrients—think of all the crumbs that wash off your plates—they're an ideal microbial habitat.

"So far we don't have any clues about how to prevent their growth," Gunde-Cimerman tells PM by email. There haven't been any reports of dishwasher-related infection, but Gunde-Cimerman wants to do much more research because the fungi could be dangerous for infants, the elderly and people with compromised immune systems.

Shower

Shower

That grimy buildup we affectionately call soap scum is actually a mat of bacteria. Warmth and moisture, two things we tend to appreciate in our showers, are also what most microorganisms look for in a habitat, as Norman Pace has shown in frightening detail.

In 2004, Pace and his colleagues scraped the slime off four vinyl shower curtains from homes in the Boulder, Colo., area and analyzed the genetic material they found there. What they saw was a rich community of bacteria that flourishes in a shower environment, but most of it was harmless. Then in 2009, Pace peered inside shower heads, where it's not only moist and warm but also dark—yet another thing bacteria love. Of 45 shower heads sampled, 20 percent were growing Mycobacterium avium, an opportunistic pathogen that can infect people with weakened immune systems. Pace also found trace amounts of common infectious agents like staphylococcus and legionella. The bugs come from the municipal water supply and multiply in the shower head's cozy confines, he says.

The cause for concern here is that about half the water droplets in a typical shower head's spray are small enough to be sucked into human airways, Pace says. And if you breathe them in, you breathe in their microbial passengers. In fact, one hypothesis proposed to explain the rise in pulmonary infections by mycobacteria over the past few decades blames our increasing preference for showers over baths, though Pace says there isn't much data to solidify that conclusion.

To him, the shower bacteria aren't cause for great concern. People can clean or frequently replace a shower curtain or install a rain-type shower head that doesn't make inhalable mist. However, people with healthy immune systems don't face much risk. "I continue to take showers," Pace confirms.